1. Field of the Invention
The present invention relates generally to pressure sensors, and specifically, to micro-machined piezoresistive pressure sensors.
2. Background Information
A conventional prior art silicon-based pressure sensor consists of a semiconductor die having a thinned region or diaphragm, a sensing means for sensing diaphragm displacement, and an enclosure with one or more pressure ports. Pressure from external sources is typically communicated to one or both sides of the diaphragm using gaseous or liquid media. Differential pressure across the diaphragm results in bending of the diaphragm, which is typically detected by means such as stress-sensitive piezoresistors strategically implanted into the silicon diaphragm.
FIG. 1a illustrates a prior art anisotropically etched silicon pressure sensor 10. The pressure sensor 10 includes a sensor die 12 having a silicon diaphragm 14 in a central region of the sensor die 12, and four strain-sensitive piezoresistors 16 placed around the periphery of the central region, as shown in FIG. 1a. Referring now to FIG. 1b, a cross-sectional view of the prior art anisotropically etched silicon pressure sensor 10 may be seen. This view shows an underlying cavity 18 formed during anisotropic etching, and slanted flanks 20 which define the diaphragm edge 24.
FIG. 1c illustrates the typical piezoresistor placement of the prior art anisotropically etched silicon pressure sensor 10 of FIG. 1a. Referring to FIG. 1c, resistors R2 and R4 are mounted parallel to their respective diaphragm edges 24, and resistors R1 and R3 are mounted perpendicular to their respective diaphragm edges 24.sub.2. The resistors are formed in a Wheatstone bridge configuration, as shown in FIG. 1d, where the resistances of resistors R1 and R3 increase in value with increased pressure P1 (FIG. 1b) and the resistances of resistors R2 and R4 decrease in value with increased pressure P1. With a constant voltage Vb or constant current applied to the bridge, an output voltage Vo of the bridge varies as a function of the applied pressure P1.